Hi @distans, More on this issue in this thread:
It’s along time since I used the w5100 shield, but my issue wasn’t caused by duplicate MAC addresses. The shield would work when plugged directly into the router, but not when plugged into a L2 switch that was then connected to the router.
It doesn’t seem that this is the issue in this case, but worth bearing in mind for the future is you’re using these Ethernet shields.
Pete.